Well, Trump is now 71 and would be 75 at the start of his next term, so the Dems don’t have a monopoly on older candidates.
The difference is that the GOP actually has some decent younger people lined up, and if Trump decides not to run again, there are some good candidates out there. The Dems, on the other hand, have nobody except these aged whack-jobs who think they’re still pony-tailed radicals and are reliving 1968.